Charge battery, start engine put lights on hi beam with heater on high. Check voltage at battery with a VOLT METER should be between 13 to 14.5 volts! Or have a repair shop do this it for you if you dont have a tester! Only way to tell how much alt. output actually is with no damage to system.
test alternator by starting engine and disconnecting neg. if car remains running then not your alternator. check your battery. maybe do for new one. rec. deep cycle for replacement. possible too if any mod.s made to sound system(upgrades may be drawing to much power for alt. to keep up with demand. run diagnostic for any other electrical issues you need to be aware of.